1. Field of the Invention
The present invention relates to an optical sheet that is used in displays and display devices such as liquid crystal display devices and illumination devices and that excels in a light convergence function and a light diffusion function, and to a method for manufacturing such an optical sheet. More particularly, the present invention relates to an optical sheet in which a side lobe effect is reduced and excellent brightness increase ratio in the desired angular direction (for example, front surface direction) is obtained by effectively utilizing the incident light that can become side lobe light, and to a method for manufacturing such an optical sheet.
2. Description of the Related Art
Lens films that collect light from a light source such as a light guide plate in the front direction or diffusion sheets that diffuse the light have been used in recent years for displays such as liquid crystal display elements or organic EL displays.
For example, in a direct backlight, such as shown in FIG. 1, that is suitable for television sets and the like, the light emitted from a light source 42 falls on a converging film (optical sheet) 41, part of the incident light is refracted and transmitted by the optical film 41, whereby the propagation direction thereof is changed, and goes out in the front surface direction, and the remaining part is reflected and returned to the light source 42. The reflected light from the converging film 41 is reflected by the surfaces of the optical source 42, a diffusion plate 43, a diffusion sheet 44, and the like, and again falls on the converging film 41.
Such a configuration features a wide dispersion of brightness distribution of the light going out from the light source and a reduced brightness of the front surface. Therefore, the converging film 41 improves the directional characteristic of the light from the light source so that the brightness in the front surface direction increases.
FIG. 2 is a cross-sectional view illustrating an optical path in such converging film (optical sheet) 41. As shown in FIG. 2, when the incident light is refracted and transmitted by the converging film 41, the light is divided into a component A that is refracted in the front surface direction, a component B that is refracted in the direction of departing from the front surface, rather than in the front surface direction, and a component C that is reflected by the surface. From among these light components, the component A goes out in the front surface direction, that is, observation direction and represents the light that is actually used. The reflected component C is diffusion reflected by the surface of the diffusion sheet or the like, the angle of incidence thereof on the converging film is changed, and part thereof is converted into the component A and goes out in the front surface direction. The repetition of such reflection converts most of the component C into the component A and increases the brightness in the front surface direction of the outgoing surface.
By contrast, the component B of the light that passed through an X portion in FIG. 2 is the light (referred to hereinbelow as “side lobe light”) that goes out at a wide angle outside the effective viewing angle of a liquid crystal display device or the like and is lost as an unused light that makes no contribution to the brightness increase at the front surface. As a result, the brightness is reduced and also a secondary image of a high brightness is observed at an angle that is very far from the direction normal to the screen, this image causing sense of discomfort.
In order to resolve this problem, it has been suggested to provide a reflective member that reflects the incident light that can become the side lobe light on the side opposite the prism surface of the prism sheet (converging sheet), so as to reuse the incident light that can become the side lobe light by such reflection (see Japanese Patent Application Laid-Open (JP-A) No. 2006-79040).
However, the description of the aforementioned document merely indicates that such reflective member has a size equal to or less than ⅔ of the prism pattern and is located within a free space of the prism pattern, and the position thereof in which the incident light that can become the side lobe light can be effectively reflected is not accurately specified. Therefore, there is a probability of not using effectively the incident light that can become the side lobe light. Further, no specific method for forming the reflective member is disclosed.
It has also been suggested to provide a light-shielding portion on the prism sheet in order to prevent the occurrence of the side lobe light (see JP-A No. 2006-119166). This patent document discloses a specific method for forming the light-shielding portion and describes that a light transmission portion and a light absorption portion are formed separately by a self-alignment method in which light falls from a specific direction and the presence or absence (light density increase) of optical paths through which the illumination light will pass is employed. Only a portion through which the light passes (portion with a high light density) is taken as a transmission portion, and the portion through which light does not pass (portion with a low light density) is taken as a light-shielding portion.
However, in this case, the light transmission portion and light absorption portion are not formed by illumination with light from the front surface direction, and the illumination is performed from a direction at an angle of 35° to 71° to the front surface direction (direction normal to the prism sheet). Therefore, such approach is unsuitable for attaining the object of converging the light in the front surface direction. One more problem is that because a light absorption layer of a black color is employed as the light-shielding portion, the incident light that can become the side lobe light is absorbed, effective light utilization is inhibited, and brightness cannot be increased.
Accordingly, reducing the effect of the side lobe light by providing a zone having an optical property different from that of the optical sheet in a portion through which the light does not pass in the optical sheet with the object of effectively using the incident light that can become the side lobe light can be considered.
However, when “reflection” is selected as the optical property, at certain angles of incidence, the incident light that can become the side lobe light goes out as a side lobe light, without being reflected, as shown by a component D in FIG. 2. Therefore, more effective suppression of the side lobe light is still needed.
Thus, an optical sheet that excels in a light convergence function or a light diffusion function, has excellent brightness increase ratio in the desired angular direction, in particular the front surface direction, and greatly inhibits the side lobe, and a method for manufacturing such an optical sheet with good efficiency and high accuracy have not yet been suggested, and further improvement and development thereof is presently needed.